Photosolubilization



United States Patent 3,407,068 PHOTOSOLUBILIZATION Roxy Ni Fan, Highland Park, N.J., assignor to E. I. du Pont de Nemours and Company, Wilmington, Del., a corporation of Delaware N0 Drawing. Filed Oct. 13, 1964, Ser. No. 403,631 6 Claims. (Cl. 96-407) ABSTRACT OF THE DISCLOSURE Photographic silver halide emulsion layers where the silver halide crystals are photosoluble and have associated therewith in greater than fog inhibiting amounts a silver merc-aptide of a mercaptoselenazole. Before exposure to actinic radiation the crystals dissolve more slowly in aqueous sodium thiosulfate solution than untreated crystals and after exposure dissolve readily in such solution.

This invention relates to photography and more particularly to new image-yielding photographic silver halide emulsion layers and to photographic elements embodying such layers.

The principal processes of photography are based on the use of colloid-silver halide emulsion layers. In the prior art processes a latent image is formed by imagewise exposure of a radiation-sensitive silver halide emulsion layer. Silver halide bearing a latent image has been developed to silver by selective reduction in these instances. In the prior processes of photography the unreduced silver remaining after development has been removed by silver halide solvents or rendered insensitive or transparent by treatment with complexing agents. Optional further treatments include intensification, reduction, toning and tinting. However, the primary or first step in image formation always has been based on the selective reduction step.

An entirely different type of photographic process has been described in assignees Blake, US. Patent 3,155,507, Nov. 3, 1964. The novel process of said application, characterized as photosolubilization, requires the use of a specially prepared silver halide emulsion layer containing a stipulated amount of an organic compound which modifies the silver halide solubility so that, in conventional silver halide solvents, said organic compound causes the silver halide grains to dissolve more slowly than normal. Such an element is given an imagewise exposure and the exposed areas can then be treated in a solution of a silver halide solvent to yield a positive, silver halide image (the silver halide remaining undissolved in the unexposed areas). As an optional additional processing step, the silver halide image may be intensified, e.g., by reduction, to convert it into a black, metallic silver image.

It is an object of this invention to provide new photographic silver halide compositions, photographic layers, and photographic elements bearing a layer of silver halide. Another object is to provide processes for making these products. A further object is to provide such products which are adapted to more versatile processes for forming silver and other images and which are simple, dependable and give results equal in quality to the prior conventional methods. A more particular object is to pro vide new compositions and elements for photosolubilization processing embodying a class of organic compounds which provide a new combination for the required alteration of the silver halide crystals. Still further objects will be apparent from the following description of the invention.

The above objects are realized in a photographic emulice sion layer comprising, before exposure to actinic radiation, light-sensitive silver halide crystals having associated therewith a silver mercaptide of a substituted mercapto compound of the formula:

where R and R, which may be the same or different, are hydrogen, hydrocarbon or substituted hydrocarbon nuclei of 1-14 carbon atoms connected through carbon to the selenazole nucleus, i.e., alkyl, aryl, alkaryl, aralkyl radicals and such radicals containing substituent groups such as nitro, halogen, e.g., chlorine and bromine, and alkoxy of 1-6 carbon atoms. Useful alkyls include ethyl, propyl, butyl and n-pentyl, and useful alkoxy radicals include the corresponding alkoxy radicals. When separate, at least one of the radicals R and R is a hydrocarbon nucleus of 3-14 carbon atoms. When R and R are taken together, they form an aromatic or alicyclic ring. The silver mercaptide is of lower solubility in water than silver chloride and the silver halide crystals so associated with the silver mercaptide dissolve more slowly in 10% aqueous sodium thiosulfate solution, at a predetermined pH, than untreated silver halide crystals.

It is preferred that the silver mercaptide be present in such amount, in terms of the ratio of its weight to the surface area of said silver halide crystals, that when admixed in such ratio with an aqueous silver chlorobromide (70/30 mole percent) gelatin dispersion, containing 10 g. of gelatin per mole of Ag and .57 mg. of Ag per ml., and said silver chlorobromide dispersion is treated with 10%, by weight, aqueous sodium thiosulfate (so that the resulting mixture contains 0.29 mg. of silver and mg. of sodium thiosulfate), at least three times the amount of silver chlorobromide remains undissolved as in a similar dispersion successively treated with 5%, by weight, aqueous sodium hypochlorite and 10%, by weight, aqueous sodium thiosulfate (so that the resulting mixture contains 0.29 mg. of silver, 25 mg. of sodium hypochlorite and 100 mg. of sodium thiosulfate), after vigorous agitation of both dispersions for 30 seconds at 25 C.

In accordance with an important aspect of the invention, R of the above formula is hydrogen and R is an unsubstituted hydrocarbon radical of 6-10 carbon atoms and has a cyclic hydrocarbon radical of 6 carbon atoms attached through a cyclic carbon of said radical to the 4-carbon atom of the selenazole ring. Suitable radicals of the latter type include cyclohexyl, phenyl, and alphanauhthyl.

Preferably, the silver halide crystals are dispersed in a water-permeable organic colloid to form a light-sensitive photographic emulsion. The selected mercapto compound can be added to the silver halide emulsion while the latter is in the liquid state or the emulsion may be coated on a suitable support and the resulting element bathed or impregnated with a solution, e.g., an alcoholic solution of the organic compound. The desired amount of the mercapto compound in the silver halide emulsion may vary with a number of factors such as the nature of the mercapto compound and the size of the silver halide crystal (and thus the surface area of the crystal per mole of silver halide).

When the mercaptan is added to the emulsion in the liquid state it is most efficiently adsorbed to the silver halide crystal by digesting the emulsion, e.g., heating the emulsion between and F. Generally the organic mercapto compound is used in a range of from 0.3 to 1.5 gram per mole of silver halide and, more preferably, from 0.4 to 1.2 grams per mole of silver halide. The optimum concentration of mercaptan is decreased somewhat, e.g., about 10%, when the emulsion is sensitized with a photographic optical sensitizing dye as disclosed in assignees copending application of Blake, Ser. No. 390,460, filed Aug. 18, 1964.

The gelatin-silver halide ratio is not crltical and may vary from 3:1 to 1:20 depending on the particular organic compound and silver halide crystals.

In an important use of the products of the invention, direct positive images are formed by a process WhlCh comprises:

(a) Exposing imagewise to actinic radiation a photosensitive layer comprising silver halide crystals treated with the mercaptan as described above,

(b) Treating the exposed layer in a solution of a silver halide solvent to remove soluble silver halide 1n the exposed image areas, thus forming a positive silver halide image, and optionally,

Washing the resulting layers. If desired, the silver halide image may be viewed directly, e.g., by projec tion (if on a transparent support) or it may be intensified by (d) Converting the residual silver halide to silver by treatment in a fogging developer, e.g., a high pH, 1- phenyl-4-methyl-3-pyrazolidone/hydroquinone developer containing iodide ion or by fogging the emulsion by exposure to light and then treating with a silver halide reducing agent, e.g., a conventional silver halide developer, and

(e) Washing the developed layer to reveal a positive silver image in the original non-exposed areas.

The imagewise solution of the exposed silver halide/ mercaptan stratum may be effected by the silver halide solvents commonly used as photographic fixing agents, e.g., sodium thiosulfate, sodium thiocyante, concentrated solutions of potassium bromide, etc. In assignees copend-- ing application Hunt, Ser. No. 388,919, filed Aug. 8, 1964, it is disclosed that more efiicient removal of exposed silver halide crystals (insolubilized by mercapto compounds) may be obtained when the fixing solution contains an inorganic cation selected from the class consisting of potassium, cesium, rubidium, thallium (I) strontium, and lead (II). These cations are also effective as additives for fixing solutions used in treating the elements of this invention.

Reduction of the treated residual silver halide may be accomplished by the use of any chemical reducing agent capable of reducing silver ion to silver metal, e.g., hydroquinone, metol, sodium hydrosulfite and stannous chloride. The function of the reducing agent may be enhanced by modifying the surface properties of the treated, residual silver halide crystals by means of alcohol, thiourea, potassium iodide, etc. The silver halide image may be toned, e.g., with sodium sulfide, sodium selenide, etc. In addition, color images may be obtained by developing the treated, residual silver halide with a primary aromatic amine color developing agent in the presence of a color coupling compound either in the developing bath or previously incorporated in the emulsion. Additionally, the silver halide image may be intensified by dye mordanting.

The present invention is not limited to a narrow class of mercaptans With which the silver halide crystals are intimately associated or may be treated in preparing the novel compositions of this invention. Instead, a large number of useful compounds as defined above can be employed and their utility can be determined readily by a relatively simple test. Essentially, the test consists of two steps, Test A and Test B. In Test A, the candidate mercapto compound must render a dispersion of silver halide crystals insoluble in a silver halide solvent, i.e., an aqueous solution of sodium thiosulfate, at some pH between 1 and 13. If the candidate compound meets the insolubility requirements of Test A, it must also meet the requirements of Test B by forming with said dispersion of silver halide crystals a reaction product which, upon treatment with an aqueous solution of sodium hypochlorite, becomes soluble when subsequently treated with aqueous sodium thiosulfate. The following practical tests are provided in further exemplification of the invention and include specific concentrations of solutions, times, etc., so that suitable mercaptans may be readily and positively identified.

TEST A A solution nearly saturated at 25 C. with a candidate mercaptan, is prepared using ethanol, acetone, dimethyl formamide, water or other suitable solvent. Depending on the solubility, a solution concentration from 0.01 to 10 percent by Weight is obtained. Twenty-five ml. of a silver chlorobromide dispersion containing 25 mg. of silver halide (calculated as silver bromide), prepared as described below, is treated with small increments (i.e., about 0.1 to 0.2 ml. at a time) of the said candidate solution under safelight conditions (Wratten 1A filter or equivalent) until the silver halide dispersion either is rendered insoluble in 10% aqueous sodium thiosulfate or the candidate is found not to cause insolubilization. Generally insolubilization will occur upon the addition of 0.05 g. or less of said candidate mercaptan, calculated as the pure compound. Compounds which must be used in substantially greater quantities than this, e.g., 1-2 g., to effect insolubilization are considered less preferred compounds. The silver halide dispersion insolubility is determined by taking a 0.5 ml. portion of the silver halide dispersion (after each incremental addition of the candidate mercaptan), adding about 0.1 to 0.2 ml. of 10% aqueous sodium thiosulfate solution and observing the turbidity after 30 seconds.

As a control, one should use 25 ml. of water to which small increments of the candidate solution are added. Half milliliter portions of the control are treated in the same manner with the sodium thiosulfate solution. The presence of visual turbidity relative to the control is sufficient to satisfy the definition of insolubility in this test.

This test may be repeated for various pH increments from 1 to 13. Although there is some optimum pH value at which the test is most sensitive, this is not a sharp maximum which must be precisely attained. Rather, it has been found that there is a fairly broad range of pH values (e.g., 2.0 to 3.0 pH units) over which the test has a satisfactory sensitivity. In practice, the silver halide dispersion might be tested without adjustment (e.g., at pH 5.0 to 7.0) and if insolubilization occurs here, Test A is completed. If there is no insolubilization, the test is repeated at a higher pH (e.g., from pH 10l3). If there is still no insolubilization, the test is conducted with the emulsion adjusted to a lower pH (e.g., about pH 1-3). Thus three different pH values represent a practical maximum numher which must be investigated to determine whether or not insolubilization will occur.

TEST B A mercaptan capable of insolubilizing a silver halide dispersion according to Test A is now ready for the next test, which again will be conducted under safelight conditions. To the above silver halide dispersion, there is added the minimum amount of a solution of the candidate mercaptan found necessary for insolubilization. Halfmilliliter samples of the dispersion containing 0.5 mg. of AgBr or 0.29 mg. of Ag are placed in two test tubes. To one sample is added 0.5 ml of Water; to the other is added 0.5 ml. of a 5% by Weight aqueous solution of sodium hypochlorite (25 mg. NaCOl). Next, there is added to both samples, 1.0 ml. of an aqueous 10% by weight solution of sodium thiosulfate mg. thiosulfate). If, after standing for up to thirty seconds, the sample treated with sodium hypochlorite clarifies (or becomes less turbid) relative to the control sample, the candidate mercaptan meets the requirements of Test B and is satisfactory for use in accordance with this invention. The chemical testing for selecting suitable compounds has been found to give absolute correlation, i.e., organic compounds which have been subjected to Tests A and B have produced without exception when tested in actual photographic emulsions, the very elfects predicted by said tests.

The silver chlorobromide dispersion referred to in the above test is a lithographic emulsion having a silver halide composition of 30 mole percent AgBr and 70 mole percent AgCl and having 20 grams of gelatin present per mole of silver halide for the steps of precipitation and ripening. This emulsion was freed of unwanted, soluble, by-product salt by a coagulation and wash procedure as taught in Waller et al., US. Patent 2,489,341, wherein the silver halide and most of the gelatin were coagulated by an anionic wetting agent, sodium lauryl sulfate, using an acid coagulation environment. Following the washing step, the emulsion coagulant was redispersed in water. Assuming a loss of grams of gelatin per mole of silver halide during washing the final gelatin concentration was about 10 grams per mole of silver halide. For use in the above test, the dispersion was diluted to the extent that one milliliter of dispersion contained one mg. of silver halide (calculated as AgBr, or 0.58 mg. of Ag).

Dispersed crystals of silver halide, treated with an appropriate amount of a suitable mercaptan are affected by exposure of a portion of said crystals to actinic radiation, e.g., ultraviolet, visible, infrared, X-radiation, etc., to such an extent that at least of the less soluble (unexposed) crystals remain when 90% of the more soluble (exposed) crystals dissolve when treated in 10% by weight aqueous sodium thiosulfate solution.

Suitable elements in this invention can be prepared by bathing a photographic film in a solution of an appmpriate mercaptan. In this embodiment, the silver halide crystals near the surface of the coated emulsion stratum are in contact with a higher concentration of the mercaptan. Crystals farther from the surface are treated with less of the mercaptan and, if the rate of diffusion is sufficiently slow, there may be considerably less of the mercaptan (even approaching zero) reacting with the lower than with the surface silver halide crystals. In such elements, satisfactory results might be obtained with only a fraction, e.g., one-half of the amount of the mercaptan theoretically calculated as required to just cover the surface of a mole of the silver halide crystals.

The invention will be further illustrated by, but is not intended to be limited to, the following example.

Example I.2-mercaptobenzoselenazole 2,2-diaminodiphenyl diselenide (17.1 g., 0.05 M) was dissolved by heating in 100 ml. of ethanol in a 3-necked round bottom flask fitted with stirrer, condenser and dropping funnel. Through the dropping funnel there was then added 50 ml. of a 24% by weight aqueous soltuion of HCl, followed by the addition of 250 ml. of hot water. About 6 g. of Zinc dust were then added slowly, stirred without further heating for 90 minutes, and the resultant solid impurities removed by filtration. Upon the addition of a 10% by weight solution of sodium acetate, the zinc salt of o-aminoselenophenol precipitated as a White solid. The salt was washed with methanol and dried to give 18.3 g., a 90% yield.

Ten g. (0.025 M) of the Zinc salt were dissolved in a solution which had been prepared by dissolving 12.5 g. of NaOH in 200 ml. of methanol. The Zn(OH) which precipitated was removed by filtration and 100 ml. of carbon disulfide was added to the filtrate. The mixture was then refluxed with stirring for three hours. Next, the excess carbon disulfide and most of the methanol were removed by vacuum distillation and 200 ml. 'of water were added to the residue. The warm mixture was filtered and the filtrate was cooled and acidified with 10% HCl. The product first formed a oily drops, then solidified. Recrystallization from ethanol-water gave 4.8 g. (92% yield) of the desired product, 2-mercaptobenzoselenazole with M.P. 160162 C.

A lithographic emulsion containing about 57 g. of gelatin per mole of silver halide (prepared as described following Test B) was diluted with water and brought to a temperature of F. The Z-mercaptobenzoselenazole prepared as described above, was added to the emulsion from a 1% ethanol solution in the amount of 0.94 g. per mole of silver halide. After digesting for 20 minutes, chrome alum hardener and other emulsion adjuvants were added. The emulsion was applied at a coating weight of 72 mg./dm. (calculated as AgBr) on 0.004 inch thick polyester photographic film base as described in Example IV of Alles, US. 2,779,684. The coating, after imagewise exposure, showed a greater rate of fixing in a 1.0 N (0.5 molar) aqueous sodium thiosulfate solution in exposed areas than in the unexposed areas so as to form a positive silver halide image. Subsequent flashing to white light, followed by treatment with a reducing agent (a conventional photographic developing solution containing 1-phenyl-4-methyl-3-pyrazolidone and hydroquinone), resulted in the formation of an intensified positive image of metallic silver.

A sample of the 2-mercaptobenzoselenazole was found to produce the required insolubilization of silver halide crystals when tested according to the above-described Test A. Chemical solubilization also occurred according to Test B as described above. More quantitative results were obtained by applying Test C, a simulated photographic test, as described below.

TEST C A 0.5 ml. portion of the insolubilized dispersion prepared in Test A under safelight conditions is placed in a 12 x 75 mm. heat-resistant glass test tube three inches from a high-intensity, tungsten filament lamp (General Electric reflector photoilood lamp ASA No. PH/RFLZ). This dispersion is exposed for up to 10 minutes. A control consisting of another 0.5 ml. portion of the insolubilized silver halide dispersion from Test A is taken under safelight conditions. Two-tenths of a milliliter of 10% aqueous sodium thiosulfate is added to each of the dispersion samples and the samples are compared under safelight conditions. Any reduction in turbidity of the dispersion exposed to the photoflood lamp compared to the unexposed control, after treatment with aqueous sodium thiosulfate solution, shows that photosolubilization occurs.

In this more quantitative test, it was determined that the approximate minimum quantity of Z-mercaptobenzoselenazole required to insolubilize 25 milligrams of silver halide was 0.0002 g.

The silver halide need not be a combination of silver chloride and silver :bromide, but may be silver chloride, silver bromide and other mixed halide systems conventional in photographic practice, e.g., silver bromoiodide. While, for rapid processing, a high silver halide to hinder ratio is desirable, more conventional ratios can also be used.

In place of part of the gelatin, other natural or synthetic water-permeable organic colloid binding agents can be used and in some cases such binders can be used alone. Such agents include Water-permeable or watersoluble polyvinyl alcohol and its derivatives, e.g., artially hydrolyzed polyvinyl acetates, polyvinyl ethers and acetals containing a large number of intralinear groups, hydrolyzed interpolymers or vinyl acetate and unsaturated addition polymerizable com-pounds such as maleic anhydride, acrylic and methacrylic acid esters and styrene. Suitable such colloids of the last-mentioned type were disclosed in US. Patents 2,276,322, 2,276,323 and 2,397,866. The useful polyvinyl acetals include polyvinyl acetaldehyde acetal, polyvinyl butylaldehyde acetal and polyvinyl sodium o-sulfobenzaldehyde acetal. Other useful colloid binding agents which can be used include the poly-N-vinyllactams of Bolton US. Patent 2,495,918, various polysaccharides, e.g., dextran, dextrin, etc., the hydrophilic copolymers in Shacklett US. Patent 2,833,- 650, hydrophilic cellulose ethers and esters, and polymers of acrylic and methacrylic esters and amides. Also, it has been found practical to treat silver halide layers on a base material in the essential absence of a binder, e.g., those prepared by chemical or vacuum deposition.

The emulsion may optionally contain any of the usual adjuvants customarily employed in silver halide systems so long as they do not interfere with the adsorption and complexing action of the organic mercaptans of the invention.

The emulsions can be coated on any suitable support,

e.g., cellulose esters, cellulose mixed esters; superpolymers, e.g., poly(viny1 chloride covinyl acetate) polyvinyl acetals, butyrals; polystyrene, polyamides, e.g., polyhexamethylene adipamide, polyesters, e.g., polyearbonates, polyethylene terephthalate/isophthalate, esters formed by condensing terephthalic acid and its derivatives, e.g., dimethyl te-rephthalate with propylene glycol, diethylene glycol, tetramethylene glycol, cyclohexane-1,4-dimethanol (hexahydro-p-xylene dialcohol); paper, metal, glass, etc.

The novel photographic compositions of this invention have numerous advantages. One advantage is the simplicity of their preparation.

Photographic processes applicable to the' compositions of the invention likewise have advantages over previously known systems based on selective reduction of exposed silver halide for forming either direct positive or negative images without resorting to the special effects and sensitizing procedures previously used for preparing such images. In addition, since direct positive image formation does not require selective reduction, this process is not limited to the use of certain photographic developing agents but may be accomplished by using a wide range of reducing agents. Many such compounds are of very low cost and can be used to form images of much higher covering power than customary, thus effecting important economies in processing, as well as greatly increasing the efiiciency of the silver image.

Another advantage of this invention is that it provides new elements for forming silver images that do not require special equipment but instead can be used with conventional equipment and apparatus. A further advantage is that the elements can be used successfully by photographic technicians and photographers of ordinary skill. A still further advantage is that the elements can be processed with conventional reducing agents, e.g., developers and fixing agents. Still additional advantages will be apparent from the above description of the invention.

I claim:

1. A photographic emulsion layer comprising, before exposure to actinic radiation, light-sensitive silver halide crystals having associated therewith a silver mercaptide of a substituted compound of the formula wherein R and R are members selected from the group consisting of hydrogen and hydrocarbon nuclei of 1-14 carbon atoms connected through carbon to the selenazole ring, at least one of R and R when separate being a hydrocarbon nucleus of 3-14 carbon atoms and when taken together constituting the atoms necessary to complete with the two carbon atoms of the selenazole ring a cyclic radical.

2. A photographic emulsion layer comprising, before exposure to actinic radiation, light-sensitive silver halide crystals having associated therewith in greater than foginhibiting amounts a silver mercaptide of a substituted compound of the formula n w R C-SH wherein R and R are members selected from the group consisting of hydrogen and hydrocarbon nuclei of 1-14 carbon atoms connected through carbon to the selenazole ring, at least one of R and R when separate being a hydrocarbon nucleus of 3-14 carbon atoms and when taken together constituting the atoms necessary to complete with the two carbon atoms of the selenazole ring a cyclic radical; said silver mercaptide being of lower solubility in water than silver chloride, the silver halide crystals so associated with the silver mercaptide dissolving more slowly in 10% aqueous sodium thiosulfate than untreated silver halide crystals at a predetermined pH, the mercapto compound of said formula being present in such amount, in terms of the ratio of its weight to the surface area of said silver halide crystals, that when admixed in such ratio with an aqueous silver chlorobromide (/30 mole percent) gelatin dispersion containing 10 g. of gelatin per mole of Ag and .57 mg. per ml., and said silver chlorobromide dispersion is treated with 10%, by weight, aqueous sodium thiosulfate (so that the resulting mixture contains 0.29 mg. of silver and mg. of sodium thiosulfate), at least three times the amount of silver chlorobromide remains undissolved as compared with a similar dispersion successively treated with 5%, by weight, aqueous sodium hypochlorite and 10%, by weight, aqueous sodium thiosulfate (so that the resulting mixture contains 0.29 mg. of silver, 25 mg. of sodium hypochlorite and 100 mg. of sodium thiosulfate) after vigorous agitation of both dispersions for 30 seconds at 25 C.

3. An emulsion layer according to claim 2 wherein the layer contains gelatin as a binding agent for said crystals.

4. An emulsion layer according to claim 2 wherein the silver halide is silver chlorobromide.

5. An emulsion layer according to claim 2 wherein R is hydrogen and R is a hydrocarbon radical of 6-10 carbon atoms and has a cyclic hydrocarbon radical of 6 carbon atoms attached through a cyclic carbon thereof to the 4-carbon atom of the selenazole ring.

6. An emulsion according to claim 2 wherein said compound is 2-mercaptobenzoselenazole.

References Cited UNITED STATES PATENTS 3,155,507 11/1964 Blake 96-'-64 3,284,206 11/1964 Blake 96-107 NORMAN G. TORCHIN, Primary Examiner.

J. R. EVERETT, Assistant Examiner. 

